My understanding is that it's usually OK (but not always ideal) to use them interchangeably. The one exception is in recipes that call for natural cocoa + baking soda, it would be a bad thing to sub dutch cocoa in there because you'd screw up the leavening (acidic natural cocoa + baking soda = gas bubbles that provide leavening). Dutch cocoa won't react with baking soda since its acidity has been neutralized.
If you really need this recipe to work perfectly though, it might be worth it to get some dutch cocoa. Dutch and natural cocoa taste a bit different when you bake with them.
agreed, i made the choc chip cookies instead ;) and will look for dutch cocoa next time i'm at the store.....